Deus Crystals

Deus Crystals were almost certainly created by the Titans and thus earned the name "God Crystals" in the Athame tongue. Deus Crystals, unbeknownst to the inhabitants of Fearann ùr, contain the souls of the dead.
See — to cast a spell, the spell must take energy from somewhere. Often times this comes in the form of taking away the caster's lifeforce. It can replenish, and some extremely minor spells can be cast without Deus Crystals without any adverse effects on the caster. Deus Crystals act as a way to harness that energy and pull from the lifeforce of deceased souls that no longer need it. Like with humans, Deus Crystals can replenish, but repeated casting or extra powerful spells can fully drain a crystal, leaving it blackened and empty-looking. A fully drained Deus Crystal cannot replenish. Some schools of magic withhold from the usage of Deus Crystals, such as Spirit Magic, yet they prepare carefully as they are aware of the effects that casting can do to them.
Deus Crystals come in all forms, from white quartz to blazing-red rubies, or verdant-green emeralds. It's said that the variety of colours correspond with the Titans that created them, but the colour does not restrict the caster from which school of magic they are performing the spell from. The only restrictions come from the caster themselves.
A slight nuance and mutation of the Deus Crystals are those created by the Athame Titan, Sapientiavectem. Cascade Crystals do not denote only one colour, and instead shine a rainbow of colours as your eyes run over them. The Crystals themselves are also stronger than normal Deus Crystals when in the hands of those that can effectively wield them. (Those born under Sapientiavectem's Influence, the Dracopuer, the Elves and the Orcs. (As well as the Fae and Felfae))
Cascade crystals naturally empower spells, making the magic cast by them stronger. They are also more durable and regenerate quicker.